Air spring struts or air springs which are clamped between the chassis and the body and which have an air spring bellows which, in turn, is fastened between an air spring cover and a rolling piston are known in a multiplicity of embodiments. The air spring is under an internal positive pressure during operation.
The air spring bellows rolls under load and in the event of spring movements, with a rolling fold being formed on the outer contour of the concentric air spring rolling piston which, in the case of an air spring strut, is fastened to a cylinder tube of the damper.
In the case of air springs in the passenger vehicle sphere, use is made of air spring bellows with as thin a wall as possible, since these ensure a high rolling comfort. In order to achieve a sufficient load-bearing capacity of the entire system with said air spring bellows, the air spring bellows or the air springs or dampers are provided with what are referred to as outer guides, namely with a tubular sleeve in the form of a “supporting corset” or supporting body surrounding the air spring bellows. For example, a thin air spring bellows with a thin light metal sleeve as the supporting body can thus be optimized to high internal pressures and therefore high load-bearing capacities with the harshness behavior being good at the same time. The outer guides here are designed and arranged in such a manner that the air spring bellows can firstly run along the outer side of the air spring rolling piston and secondly on the inner surface of the outer guide.
However, this has the disadvantage that, in contrast to thick-walled and therefore more robust air spring bellows in the utility vehicle sphere, such air spring bellows react very much more sensitively to deposits on the rolling surfaces or even foreign bodies to be rolled over. The comfortable air spring bellows very easily reacts to the rolling over of deposits or foreign bodies with a high degree of abrasion or destruction of its delicate strength support, which may result in the air spring failing.
In order to counter this, use is made of an expansion bellows, in particular in the case of externally guided air springs, in order to protect the air spring bellows from dirt accumulations (for example dust, sand and gravel from the road during the driving mode), as is known, for example, from DE10302495A1, which is incorporated by reference. However, the use of an expansion bellows gives rise to the problem that, in the case of spring movements of the air spring, the air volume of the space surrounded by the expansion bellows changes at the same time and therefore a gas exchange between the air trapped by the expansion bellows and the soiled air has to take place on the outside.
For this purpose, for example, the air spring device known from DE102009003829A1, which is incorporated by reference has ventilation in the form of ventilation bores in a collar of the outer guide, wherein, in the case of the described overhead position of the air spring device, the collar of the outer guide faces the chassis. The ventilation of the expansion bellows therefore takes place at the foot of the expansion bellows.
In the case of an air spring strut, the connection of the expansion bellows on the side facing away from the carriageway customarily takes place in as fixed and tight manner as possible such that only little gas exchange if any at all takes place here. This is intended to ensure that no dirt can enter the expansion bellows interior from above and the dirt which has entered by being sucked in can pass to the outside again in the direction of the chassis.
The arrangement of the expansion bellows ventilation at the expansion bellows foot, which faces the chassis, leads to the situation in which, in the event of spring deflection movements of the air spring, because of the associated reduction in volume in the expansion bellows interior, the air is ejected downward in the direction of the chassis, and at the same time the air spring bellows on the outer guide releases an increasing region of the contact surface and covers a potentially soiled region on the air spring rolling piston.
During rebound movements of the air spring, because of the increase in volume in the expansion bellows interior, potentially soiled air is sucked in from below, at the same time the air spring bellows on the outer guide covers an increasing and potentially soiled region of the contact surface and exposes the contact surface on the piston.
It is therefore disadvantageous that, in the event of spring movements, dust and/or foreign bodies are sucked into the interior of the expansion bellows. This is problematic if said material does not pass again to the outside, but rather gets into the rolling region of the air spring bellows, as a result of which the expansion bellows has lost its required protective function and damage to the air spring bellows is not ruled out.